The prior U.S. Pat. Nos. 4,146,830 and 4,163,933 were examples of patents disclosing battery chargers utilizing a step-wise adjustment of the charging current, for example, in response to battery voltage. U.S. Pat. No. 4,217,533 utilized a trickle current after the battery was charged. U.S. Pat. No. 4,246,529 disclosed a battery charger with a constant charging current during each cycle of the 50 hertz voltage wave. U.S. Pat. No. 3,887,858 utilized a high rate constant current for initial battery charging and then utilized a timed overcharged interval. U.S. Pat. No. 3,911,349 disclosed a battery charger wherein the charging current decreases as the time rate of change of the battery voltage decreases. U.S. Pat. No. 3,938,019 disclosed a battery charger with a large constant current and then constant current pulses. U.S. Pat. No. 3,688,117 disclosed a battery charger with a high charging rate and then a medium charging rate to two different voltages per cell.
Problems with the prior art apparatus include the fact that batteries manufactured by different manufacturers behave differently on charge and have different discharge characteristics. Also the temperature and age of the battery and prior service conditions affect the rate at which a battery may effectively be charged. Batteries of small and large capacity also behave differently under a given rate of charging current. If the battery is old rather than new, it would need to be charged for a longer period of time at the same ampere rate than a new battery of the same ampere hour capacity. Further under a given charge rate, a battery may not be fully charged if charged only for a certain length of time.